Dielectric films are widely used throughout both the electronics and coatings industries. Due to their relatively high dielectric constants and melting points, there is an increasing interest in forming dielectric layers from parylene polymers having the molecular structure: ##STR1## wherein X is typically a hydrogen atom or a fluorine atom.
Parylene polymers are usually formed by chemical vapor deposition processes. One such process is the Gorham process in which a parylene dimer have the molecular structure: ##STR2## is vaporized and the dimer bonds are then cleaved to yield parylene monomers. The parylene monomers are deposited onto a surface and subsequently polymerized. Because the dielectric constant and melting temperature of parylene polymers usually increases as the number of fluorine atoms within the polymer increases, it is desirable to use octafluoro-[2,2]paracyclophane (hereafter "AF4") having the molecular structure: ##STR3## as the parylene dimer.
TFPX-dichloride having the following structure: ##STR4## is one preferred starting material for the preparation of AF4. Heretofore, the only useful preparation of TFPX-dichloride has been via a high yield, photo-induced chlorination of .alpha.,.alpha.,.alpha.',.alpha.'-tetrafluoro-p-xylene, (hereafter "TFPX"), having the following structure: ##STR5##
Unfortunately, however, all currently available methods for preparation of TFPX are expensive and/or produce undesirable amounts of hazardous waste. Additionally, known methods for production of TFPX are not readily adapted to industrial-scale processes.
The conventional procedure for synthesizing TFPX involves the fluorination of terephthaldehyde, which has the molecular structure: ##STR6## SF.sub.4 and MoF.sub.6 are the most commonly used reagents for terephthaldehyde fluorination. However, SF.sub.4 and MoF.sub.6 are expensive, reducing the industrial utility of this synthetic scheme. In addition, SF.sub.4 and MoF.sub.6 are toxic materials, so a large amount of hazardous waste is produced using these reagents.
Russian Patent No. 2,032,654 discloses an alternate method of synthesizing TFPX in which .alpha.,.alpha.,.alpha.',.alpha.'-tetrabromo-p-xylene (hereafter "TBPX") having the molecular structure: ##STR7## is reacted with SbF.sub.3 to produce TFPX. This method employs the well established electrophilic catalyzed S.sub.N 1 reaction mechanism for replacement of benzylic halogen atoms of the TBPX with fluorine atoms. According to this method, the antimony atom in SbF.sub.3 acts as an electrophile which removes bromine from TBPX to form a carbocation. The carbocation subsequently reacts with fluorine to form TFPX. While this reaction is reported to provide a good yield when carried out under comparatively mild reaction conditions, antimony-containing compounds are highly toxic and expensive. Furthermore, the SbF.sub.3 is used in a stoichiometric amount rather than a catalytic amount, resulting in large quantities of hazardous waste materials. Therefore, this method of synthesizing TFPX has limited use for industrial applications.
Finally, TFPX has been synthesized by the reaction of nucleophilic fluorine molecules with non-fluorinated tetrahalo-p-xylenes via S.sub.N 2 type nucleophilic displacement reactions. In such synthesis, the benzylic halogen atoms of a non-fluorinated tetrahalo-p-xylene are replaced by the fluorine atoms in a nucleophilic fluorine molecule without formation of a carbocation intermediate. Examples of such reactions include the reaction of .alpha.,.alpha.,.alpha.',.alpha.'-tetrachloro-p-xylene with CsF or KF. This procedure, while being less expensive and producing less hazardous waste has recently been found to be very difficult to achieve on an industrial scale.
Despite the various methods for synthesizing TFPX, which is an excellent precursor of TFPX-dichloride, a need still exists for a simple, environmentally friendly process for making TFPX-dichloride which utilizes inexpensive, readily-available reactants, and which is scaleable to an industrial level of production.